Polycarbonate generally has poor impact resistance for thickness in excess of 1/8 to about 1/2 inch. The thickness effect can be explained by considering the stress state as behind a notch. In thin samples, the material behind the notch is subject to plain stress and can participate freely in shear, resulting in great toughness. In thick samples, the material behind the notch exhibits plain strain, inhibiting the outset of shear deformation but facilitating the onset of crazing; this results in brittle behavior. Heretofore, impact modifiers which were utilized to improve the impact resistance of polycarbonate included compounds such as butadiene-styrene copolymers which were coated with a copolymer made from styrene, methyl methacrylate, acrylonitrile, or butyl acrylate which was coated with methyl methacrylate, and the like.
A number of patents exist with regard to impact modifiers for various polymers and include the following:
U.S. Pat. No, 3,251,904 to Souder, et. al., relates to solid, thermoplastic, polymeric products from the sequential polymerization of alkyl methacrylates with polyalkyl acrylates and to high impact resistance materials resulting from blends of such products with other polymeric materials, especially polyvinyl chloride.
U.S. Pat. No. 3,655,825 to Souder, et. al., relates to solid, thermoplastic, polymeric products resulting from the polymerization of lower alkyl esters of acrylic and methacrylic acid and to high impact-resistant materials which result from blends of such products and other polymeric materials, particularly polymers and copolymers of vinyl chloride. Crosslinking monomers are utilized.
U.S. Pat. No. 3,793,402 to Owens relates to an impact resistant thermoformable composition having improved stress whitening behavior comprising a blend of a thermoplastic polymer and a multi-stage, sequentially produced polymer characterized by a non-elastomeric hard stage, an intermediate elastomeric stage, and a relatively hard stage.
U.S. Pat. Nos. 3,808,180 and 3,843,753 to Owens relates to rigid thermoplastics such as acrylic polymers and vinyl halide polymers which are modified to impart high impact resistance.
U.S. Pat. Nos. 3,859,384 and 3,859,389 to Carty, et. al., relate to acrylic modifiers for vinyl halide polymers having superior mill roll release at high temperature and resistance to "plate out," obtained by modifying the polymers with a multi-phase based acrylic composite polymer having a first non-crosslinked phase and a final, rigid thermoplastic phase polymerized in the presence of the first phase.
U.S. Pat. No. 4,086,296 to Carty, et. al., relates to blends of thermoplastic polymers and a multi-phase acrylic composite polymer comprising a non-crosslinked phase, and a final rigid thermoplastic phase polymerized in the presence of said first phase. The core polymer has a very low molecular weight, that is less than 50,000, and contained no suggestions of imparting improved impact resistance.
U.S. Pat. No. 4,096,202 to Farnham, et. al., relates to a blend of a polyester and an impact modifier which is a multi-phase composite interpolymer comprising a crosslinked acrylic first stage and a final rigid thermoplastic stage.
U.S. Pat. Nos. 4,299,928 and 4,378,449 to Witman relate to polycarbonates having improved impact performance comprising a blend of a polycarbonate resin and a C.sub.1 to C.sub.6 acrylic rubber interpolymer composite. These patents also utilize a crosslinker as well as a graft linker to bond the various phases or shell and core together.